Process for preparing biaxially oriented polypropylene film



P. S. BLATZ April 12, 1966 2 Sheets-Sheet 1 Filed Aug. 17, 1962 FIG.

INVENTOR PHILIP S. BLATZ ATTORNEY A ril 12, 1966 P. s. BLATZ 3,246,061

PROCESS FOR PREPARING BIAXIALLY ORIENTED POLYPROPYLENE FILM Filed Aug. 17, 1962 2 Sheets-Sheet 2 INVENTOR PHILIP S. BLATZ BYJMM "'TAITORNEY United States Patent 3,246,061 PROCESS FER PREPARING BIAXIALLY ORIENTED PGLYPRBPYLENE FILM Phiiip S. Blatz, Wilmington, Del., assignor to E. 1. du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware Filed Aug. 17, 1962, Ser. No. 217,638 3 Claims. (Cl. 264-95) This invention relates to a novel process for preparing polypropylene film and in particular to a polypropylene film that is durable and is, without a special coating, heatsealable.

By heat-scalable is meant the ability of the film to produce a seal that will withstand a force of at least 1,000 grams for a one-inch sample, the seal having been made by subjecting two overlapping thicknesses of the film to a pressure of psi. with a' conventional sealer at a temperature of 200 C. for a dwell time of 1.5 seconds.

By durable is meant the ability of the film to withstand at least 90 grams/mil of thickness when subjected to the dart-drop test as described in ASTM-D1709-59T.

By polypropylene films are meant films prepared from the polypropylene homopolymer, cop-olymers wherein propylene is the major cornonomer and blends of polypropylene with other polymers, e.g. blends with polyethylene wherein polypropylene or a copolymer thereof is the major constituent. Such films may also contain additives such as plasticizers, dyes, fillers, pigments, slip agents, etc., that do not detract from the basic characteristics of the polypropylene film.

In the development of polypropylene film, it has been found that satisfactory durability may be provided by biaxially orienting the polypropylene film. Biaxial orientation may be accomplished by stretching fiat films in two mutually perpendicular directions or by expanding tubular film in combination with stretching the tubular film longitudinally. However, polypropylene films that have been stretched biaxially, while being very tough and durable, become very high melting. Consequently, they must be coated with materials that will melt to provide heat-seals, the heat-seals being absolutely essential.

in the making of bags. It would be desirable to provide a polypropylene film that is both durable and heat-scalable without a coating. Such a process has now been found.

It has now been found that only by biaxially stretching polypropylene film in a particular manner can one produce the combination of durability and heat-scalability. Specifically, the polypropylene film must be extruded as a molten tubing at a temperature that is usually above 200 C., but not above 275 C., preferably ZOO-225 C.; advancing it over but out of contact with a cooling surface, the cooling surface consisting of at least two sections: a frusto-conical section, the narrow end of which faces the point of extrusion; and, continuous therewith, a cylindrical section, the frusto-conical section serving to cool the film from the molten state to a temperature within the orientation temperature range, the cylindrical section serving to complete cooling of the film to a temperature of -50 C., the annular surface of the frustro-conical section making an angle of -35 degrees with the annular surface of the cylindrical section, i.e. the angle of the cone is 25-35 degrees, preferably degrees; expanding the tubing to a diameter of at least 1.5 times, preferably 1.5-3.0 times, its diameter as extruded while ad- "ice vancing it over the cooling surface; and advancing the tubing at a rate of 10-80 times the rate of extrusion.

In the following table, Table I, the orientation temperature range, the density and the crystalline melting point are listed for some representative propylene polymers.

The polypropylene film produced by the process of this invention is heat-scalable and durable. structurally, it has a crystalline habit as measured by X-ray diffraction that is substantially of the delta form. It has an orientation ratio greater than 2/1 and usually not greater than 15/1 and its orientation angle is between 25 and degrees.

The invention will be more clearly understood by referring' to the drawing and examples which follow. In the drawing, FIGURES 1 and 2v are partially schematic views in longitudinal section of apparatus for performing the invention.

Referring to FIGURES 1 and 2, the molten polypropylene is ejected from the die 10 as a tubular film 11. The temperature of the molten resin may be anywhere from 200 C. to 275 C. However, at temperatures above 250 C. it is necessary to use an external air quench at the outlet of the die. This may take the form of a ring surrounding the tubular film, the ring having openings through which cool air may be blown onto the surface of the film.

The tubular film is next advanced over a cooling hell or mandrel 12 by a pull roll set 13. The cooling bell 12 is composed of at least two sections: a frustro-conical section 12A and a cylindrical section 12B. It is preferred to operate with the mandrel 12 disposed as close to the die 10 as possible. The total length of the cooling mandrel will depend upon the final temperature desired for the film, the film speed and the heat transfer efiiciency of the particular equipment used. However, the ratio of the frustro-conical section to the cylindrical section should be from 1/2 to 2/1 and the length of the frustro-conical section should be at least half the distance from the die face to the beginning of the cylindrical section, i.e. to the knuckle between the conical and cylindrical sections.

Another critical variable is the surface characteristic of the cooling mandrel, especially at the annular juncture of the frustro-conical section with the cylindrical section. At this point, the surface roughness should be at least 50 microinches based on arithmetic average deviation from the mean height of the surface. At surface roughness values less than this, the extruded film has a tendency to stick to the surface of the cooling mandrel. At higher values, for example, greater than 500 microinches, protrusions from the surface of the mandrel tend to scratch the interior surface of the extruded film. This surprising finding is believed to be due to the propagation of chance contacts. It is theorized that when the surface of the mandrel is too smooth, i.e. less than 50 microinches, occasional contact of gel particles in the film with the man- 3 drel tends to. propagate. rapidly to cause sticking. of a wide area of film. Of course, if the surface of the mandrel is too rough, then the protrusions scratch the film.

Cooling may be accomplished by circulating cooling water through tubes set in the surface of the cooling bell conical section 12A with the final cooling to room temperature taking place in the cylindrical section 12B.

Another important and critical consideration is the angle between the frustro-conical section and the cylindrical section'designated in FIGURES 1 and 2 as a. It has been found that this angle must lie between 25 and 35 degrees to obtain the polypropylene film of this ingention. If the increase in diameter of the film is more gradual, i.e. the angle is less than 25 degrees, thenthe final film may be durable but its heat-seal is inadequate. If the angle ix is greater than 35 degrees, then, irrespective of the rate of cooling or the increase in diameter of the tubing or the curvature at the knuckle, the film tends to stick to the cooling mandrel. It should also be mentioned that the 7 density of the polypropylene aiiects angle a. The higher The lip opening of the die 4 was 20 mils. The output of the resin was 40 lbs./hour. The film speed was 40 feet/minute. The diameter of the cooling mandrel was 12 inches. The distance of the die to the knuckle of the cooling mandrel was 12 inches. The angle of the surface of the irustro-conical section with the annular surface of the cylindrical section was degrees. The extruded tube was slit into two fiat sheets which were wound on separate rolls.

The physical properties of the listed in Table II.

TABLE II film so produced are Modulus, p.s.i. Tear, gram/mil- Heat-seal strength, gram/ Dart drop, grams/mil Haze, percent Transparency, percent Gloss, 20 Orientation rati o Orientation angle, dc". Crystalline habit Description of Tests The heat-seal test is carried out by sealing two one s inch wide strips offilm to each other at one end of the shown in FIGURE 1. In this figure the air is passed in through tube 19A and the amount of air used to expand the film may be controlled by controlling the amount of air out through tube 19B. This particular method of expanding tubular film is described in greater. detail in U.S. Patent 2,987,767.. 7

, Another method for expanding the tubular film is that shown in FIGURE 2. In this apparatus, air is admitted through tube 20A and forms a bubble of the tubular film between the face of the die and the pull rolls. Excess air to control the size of the bubble and the expansion of the tubular film is bled from the system through tube 20B. This particular method is described in greaterdetail in US. Patent 2,966,700. In either case, it has been found to becritical for theresults of this invention'toexpand t-he film by gas pressure to the extent of at least 1.5 times 7' .the extruded diameter and preferably from 1.5 to 3.0

, times the extruded diameter. 7

The amount of longitudinal stretching is controlled by the speed of the pull roll set 13 relative to the speed of extrusion. For the purpose of the present invention, it is preferred that the speed of the pull rolls be such as to V produce film properties in the longitudinal direction that are substantially the same as those produced by the expansion in the transverse direction. In the operation of this process, the total drawdown due to the relative rates of the pull rolls and extrusion is between 12 times and about 15, times. However, much of this drawdown ocours on the molten resin with a minimum elfect on orienta- Example I The equipment for carrying out this example is shown schematciallyin FIGURE '2'. Tenite polypropylene resin No. 4240 (obtained from Eastman Chemical Products) was extruded through a 3% inch diameter extruder and through a circular die of 8 inches diameter at 215 C.

strips at a temperature of 200 C., a dwell time of 1.5 seconds and a pressure of the sealing bar of 10 p.s.i. .The unsealed ends are placed in a Suter tester and pulled apart. The highest force in grams required to pull the strips apart is taken as a measure of the heat-seal bond strength.

Modulus or initial tensile modulus in p.s.i. is directly related to film stiffness. It is the shape of the initial or Hooltian portion of the stress/strain curve, the film being elongated at a rate of per minute and the value being based on the initial cross-sectional area of th sample.

Tear strength is measured according to the procedure of ASTM68944.

Gloss is the term used for the quantity of light reflected The dart drop test is carried out according to the" procedure described in ASTM-D1709-59T.

Haze is measured according to the procedure of ASTM D1003-59T.

Crystalline .habit was determined by X-ray diifractiom The crystalline form characterized by two maxima, when 20 is 15.3 and 21.5 is designated the delta form. This appears similar to the crystalline form described by Asby and Hoeg, Journal of Polymer Science, 39, 535 (1959). The crystalline 'form having four maxima, when 29 is 14.2, 17.2, 18.6 and 2l.8, is designated the alpha form. The alpha form appears essentially monoclinic and similar to the crystalline form described by Natta and coworkers v(Atti accad. naz. Lincei. Rend Classe sci. fis. mat. e nat. 21 365 (1956), with unit cell dimensions a -6.54 A., alpha 95 b=7.47, beta=l14.6, 0:10.56, gamma=1l2.5.

To determine orientation ratio and orientation angle,

the films were first converted to the alpha form by heating the films clamped in frames for 10 minutes at C.

In this form orientation could be determined conveniently. X-ray difiraction patterns of the film were obtained using the pole-figure technique.

The X-ray diflractions were carried out on a-General,

described in Chapter VIII of the brochure Single Crystal' Orienter-Manual No. 12130 A, prepared by Dr. Thomas C. Furnas, in, General Electric Co.

Orientation ratio is a measure of the orientation of the ordered regions in the film. For purpose of this descrip tion, it is defined as the ratio of the diffraction intensity 6 Example 3 A polypropylene resin (Avisun 1014) containing 10% by weight of a 50/50 ethylene/ propylene copolymer having a melt index of 0.5 was prepared using the process of 9 f ms rodued re of the crystallographic planes in the plane of the film Example The Properties 0 the fil o P c we as follows. minus the minimum diffraction intensity at any diffraction angle divided by the diffraction intensity of the gloss-l0. crystallographic planes of the edge of the film minus the Transparen y, perceneefiiiminimum diffraction intensity at any difiraction angle. 10 Haze, percent2l7.0.

Orientation angle is an indication of the degree of Dart drop (-gram/un1t)600+. alignment of the polymer chains in the crystalline portion Crystalline habitDelta. of the sample in a direction parallel to the major plane Orientation ratio-9/ 1. of the film specimen. It is obtained also from the X-ray Orientation angle-. diffraction pattern. For the samples examined herein, the 15 Heat-seal strength, grams/iuch-Greater than 1000. ggienltzsitgon angle was obtained using the peak intensity at Examples The sample is rotated through the angular range of chi The procedure of Example 1 Was repeated substanwith the intensity of the X-rays diffracted being monitially using several different polypropylene resins as shown tored. The orientation angle is measured in degrees of in Tabl IV.

TABLE IV Die Rate, Film Air to Distance External Ex- Resin temp, lbs/hr. speed bubble die to air ample C. (c.f.n1.) knuckle, quench inches Escon 114 275 41 36 0.3 14% On. Tenite4240 210 35.4 40 0.3 12 on. Prorawenoflm 263 40.2 36 .05 14% On. Avisuu1067x 250 35.4 40 0.3 12 on.

the line halfway between the base and the maximum of the peak parallel to the base and intercepted by each The properties and the structural characteristics of the resulting film are shown in Table V.

TABLE V Modulus Tear, Heat-Seal Dart Gloss Haze, Trans- Orien- Orien- Crystalline Example Resin K, p.s.i. .lrnil Strength, Drop, 20 percent parency, tation tation Habit MD/TD IvlD/TD g./in. g./mil percent Ratio Angle,

Escon 114 72/68 41/98 1 1,000 144 86 1.9 72 5/1 88 Delta. Tenite 4240 66/76 52/67 1 1,000 178 125+ 0. 8 85 3. 5/1 122 D0. Profax 6420 82/78 39/55 1 1, 000 147 105 1. 9 78 10/1 100 D0. Avisun 106 81/79 42/78 1 1,000 103 133 0. 6 76 4. 7/1 D0.

1 Greater than.

end of the curve. The general procedure follows that described by H. G. Ingersoll, J. Applied Physics, 17, 924 (1946).

Example 2 A series of extrusion runs Was carried out using several different extrusion methods to illustrate the efiects of such different extrusion methods on the properties of the extruded films. A polypropylene resin (Avisun 1014 from Avisun Corp.) having a melt index of 1.1 was used for all of the extrusions. The results of these runs are shown in Table H1. range, a second section continuous with the first section TABLE III 20 Percent Percent Dart Drop Orien- Orien- Heat-seal Run no. Process Crystalline habit Gloss transhaze (gins/mil) tation tation Strength parency Ratio Angle, (gins/in.)

degrees A Same as Example 1 Delta 105 85 1. 3 170 10/1-I- 1 1, 000 B Flat diewater quench as in U.S. Alpha and delta- 63 67 0.8 y 20 1. 5/1 36 1 1,000

Patent 2,956,306. C Blown as in 13.8. Patent 2,461,975.-" Alpha 5 16 19. 8 24 1. 4/1 18 1 l, 000 D Flat diecl1illroll quench as in U.S. el 77 3. 6 55 1. 8/1 44 1 1, 000

Patent 2,364,435.

1 Greater than.

and from 0.5 to 2 times the length of said first section being a cylindrical section serving to cool the film to a temperature at which said film is set, the annular surface of the first section making an angle a of 25-35 degrees with the annular surface of the second section; expanding the tubular to a diameter of at least 1.5 times its diameter as extruded While sa-id film is advanced over the cooling surface; and adwancing the tubular film at a rate of at least 10 times the rate of extrusion.

2. A process as in claim 1 wherein said angle a is 30 degrees. 1 i

3. A process as in claim 1 wherein said tubular film is expanded to a diameter of 1.5-3.0 times its diameter as extruded.

7 References Cited by the Examiner UNITED STATES ,PATENTS' ROBERT F. WHITE,

Primary Examiner.

ALEXANDER H. 'BRODMERKEL, Examiner.

J. R. DUNCAN, M. H. ROSEN,

A. R. NOE, Assistant Examiners. 

1. A PROCESS FOR PREPARING BIAXIALLY ORIENTED POLYPROPYLENE FILM WHICH COMPRISES EXTRUDING A POLYPROPYLENE RESIN IN THE FORM OF A MOLTEN TUBULAR FILM AT A TEMPERATURE OF 200-275*C.; ADVANCING SAID MOLTEN TUBULAR FILM OVER BUT OUT OF CONTACT WITH A COOLING SURFACE, THE COOLING SURFACE CONSISTING OF AT LEAST TWO SECTIONS, A FIRST SECTION BEING A FRUSTO-CONICAL SECTION, THE NARROW END OF WHICH FACES THE POINT OF EXTRUSION, SERVING TO COOL THE FILM TO A TEMPERATURE WITHIN ITS ORIENTATION TEMPERATURE RANGE, A SECOND SECTION CONTINUOUS WITH THE FIRST SECTION AND FROM 0.5 TO 2 TIMES THE LENGTH OF SAID FIRST SECTION BEING A CYLINDRICAL SECTION SERVING TO COOL THE FILM TO A TEMPERATURE AT WHICH SAID FILM IS SET, THE ANNULAR SURFACE OF 